Turbine blades or vanes of gas turbines or steam turbines are typically exposed to severe mechanical, chemical and thermal loading. For this reason, the surfaces of the respective airfoil and the surfaces of the blade or vane platforms, which are likewise exposed to said loading, are provided with suitable coatings. A coating system of this type comprises, for example, what is termed a bond coat and a thermal barrier coating (TBC).
The airfoil of a turbine blade or vane is generally arranged on what is termed a platform. The platform and the blade or vane root arranged therebeneath serve to arrange and to fasten the turbine blade or vane on a mount component, for example a rotor disk or a ring or blade or vane ring. In this context, it is undesirable for coating material resulting from the coating of the airfoil and the surface of the platform to protrude beyond the edge of the platform and to be located in particular on the lateral face of the platform. Corresponding contamination of the lateral faces of the platform would make it more difficult to precisely arrange the turbine blade or vane on the mount component, for example on the rotor disk or on a ring, and could moreover lead to undesirable mechanical effects or leakage flows.
The airfoil and the surface of the platform are preferably coated by spraying on the coating material. Possible overspraying of the platform edges of turbine blades or vanes, for example guide vanes or rotor blades, is not acceptable, and therefore, for the aforementioned reasons, coating material which has passed beyond the platform edge has to be removed manually, for example has to be abraded. This results in increased costs, an additional loss of time during the production process and, under certain circumstances, undesirable deviations in the dimensioning of the respective turbine blade or vane.
In order to avoid the above-described post-machining of the platforms, the marginal edges of the top surface of the platform are partly set back by recesses, such that the transition between the top platform surface and the lateral face of the platform has a stepped configuration. A step or recess of this type usually has a height of 4 mm.
A slot for a sealing strip is generally arranged on the lateral face of the platform. The slot comprises a top and a bottom edge. Since the platform of rotor blades or guide vanes becomes thinner in the direction of the generator connected to the turbine, or in the main direction of flow, this being due to the contour of the hot gas flow duct, there is the risk that the top edge becomes continuously thinner, and under certain circumstances will disappear, owing to the above-described step or recess. The top edge of the sealing slot is required, however, in order to prevent coating material from being sprayed into the slot itself.